In a wireless communication system, a data packet format usually includes control information and user data. The control information includes data such as source and destination addresses, error detection codes like checksums, and sequencing information. Control information may be found in packet headers and trailers, and user data is included in between. The control information further includes a fixed-pattern preamble. The preamble serves several purposes, namely, to identify a packet type, and to provide a signal for Direct Current (DC) offset estimation, frequency offset estimation and channel estimation, for example.
In addition, another example purpose of the preamble is to allow time for a receiver to achieve lock of a receiver digital phase lock loop that is used to synchronise a receive data clock to a transmit data clock. At a point when a first bit of the preamble is received, a receiver may be in an arbitrary state (i.e., have an arbitrary phase for a local clock of the receiver). During the course of the preamble, the receiver learns a correct phase, but in so doing, the receiver may miss (or gain) a number of bits. A preamble thus usually includes a predetermined pattern to mark the last two bits of the preamble. When the pattern is received, the receiver begins collecting bits into bytes for data processing. The receiver may also confirm a polarity of a transition representing a logic high data bit to the receiver (as a check in case the bit has been inverted), for example.
Different communications protocols use different conventions for distinguishing between control information and user data. In Binary Synchronous Transmission, for example, the data packet is formatted in 8-bit bytes, and special characters are used to delimit different elements. Other protocols, like Ethernet, establish a beginning of a header and data elements by a location relative to the start of the data packet. Some other protocols format information at a bit level instead of a byte level.
Many over-the-air modulation techniques use similar basic protocols, and each technique usually includes use of preambles for use in transmission of data as well as management and control of wireless links. Transmitters and receivers may be programmed and designed to receive wireless signals, and to decode the signals based on expected preamble content. A signal transmitted by the transmitter can be affected by channel errors, and consequently, a preamble sequence may be altered during transmission. Errors in identification of a correct preamble sequence can lead to problems at a receiver that decodes system information.